The toilet in my house exhibits the same problem as one of the earlier posts, it does not always flush all of the contents in the bowl. I did everything you suggested and found no ubstruction in the toilet itself. While I had the toilet out in the yard I placed it across a couple of 2x4's and filled the tank several times. It would flush properly every other time. Feeling that this was an improvement and being at a loss as to what to do next, I reinstalled the toilet. It is now back to it's old behavior, flushing properly perhaps one in ten times.The drains in the house all work fine. I checked the stink pipe to see if there was an ubstruction there, no. I ran large volumes of water down the toilet drain...no problems. The toilet is not that old...about twenty years...and seems to be in fine shape. I also replaced the fill valve...the old one was working but looked as though it was on it's last legs. What next?